1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member capable of preventing interference. The invention also relates to an electrophotographic apparatus incorporating the electrophotographic photosensitive member.
2. Description of the Related Art
In general, an electrophotographic process includes the steps of charging a photosensitive member, exposing the photosensitive member to an image so as to form an electrostatic latent image on the photosensitive member, and developing the latent image by a developer so as to make the image visible. This process is widely used in copying machines for producing copy images on ordinary paper sheets. Hitherto, several types of electrophotographic processes were known and used, such as an electro-fax process, a xerographic process and an NP-type process which is, for example, disclosed in Japanese Patent Publication No. 42-23910.
The electro-fax process and the xerographic process make use of the so-called "Carlson" process for forming an electrostatic latent image. More specifically, each of these two types of electrophotographic processes uses a photosensitive plate composed of a carrier sheet and a photoconductive layer of zinc oxide, OPC (organic photoconductor), amorphous selenium or amorphous silicon formed on the carrier sheet. The surface of the photosensitive plate is uniformly charged and is exposed to light from an original so as to attenuate the electrostatic charge in the regions irradiated with light, whereby an electrostatic latent image having a light or dark pattern corresponding to that of the original, is formed. The thus-formed electrostatic latent image is developed by charged colored particles so as to become a visible image, which image is then either fixed directly to the photosensitive member or, is fixed after transfer to another image carrying member, such as copy paper. In any event, a fixed electrophotographic image is obtained.
On the other hand, the NP-type process forms an electrostatic latent image by utilizing the photoconductivity of a photoconductive layer and also the difference in the electrostatic capacitance between the photoconductive layer and an insulating layer provided on the photosensitive layer. After the forming of the electrostatic image, the developing, the transfer and the fixing steps are sequentially executed to produce a fixed electrophotographic image, as in the case of the electro-fax and xerographic processes.
Various electrophotographic copying apparatuses making use of these electrophotographic processes have been developed and used.
In general, known electrophotographic apparatuses employ photosensitive members in the form of a cylinder or an endless loop of web with the photosensitive layer formed on its outer surface. Conventionally, charging and exposure are effected from the outer side of the photosensitive member. In recent years, however, apparatuses have been proposed which employ a photosensitive member having a light-transmissive carrier so as to enable the imagewise exposure to be performed from the inside of the carrier in order to attain a compact electrophotographic apparatus and to simplify the process. The exposure through the carrier of the photosensitive member is, for example, an imagewise exposure, pre-transfer exposure, pre-cleaning exposure or charge-removing exposure.
The pre-transfer exposure, pre-cleaning exposure and the charge-removing exposure are conducted in such a manner as to uniformly expose the entire area of the photosensitive member. Such exposure is effected by applying a light from a suitable light source, such as a fluorescent lamp, halogen lamp or a tungsten lamp, which is disposed inside the photosensitive member, through a slit which enables the light to reach only the region which requires the exposure. Alternatively, the exposure is conducted by using a laser light source or an LED array capable of illuminating only the region requiring the exposure. Thus, pre-transfer exposure, pre-cleaning exposure and charge-removing exposure can be conducted by light applied from the space inside the photosensitive member, without substantial difficulty.
In contrast, an image exposure from the space inside the photosensitive member encounters the following problem. When the copying apparatus is of the so-called analog copying apparatus type in which the photosensitive member receives light reflected from an original to be copied, a complicated and large-size arrangement is necessary for applying the reflected light to the photosensitive member from the space inside the photosensitive member. In this type of electrophotographic apparatus, therefore, it is quite meaningless to conduct the imagewise exposure from the interior of the photosensitive member. For this reason, the imagewise exposure from the interior of the photosensitive member is applicable only to apparatus of the digital type, in which a laser or an LED light source array is applied digitally in accordance with a digital signal obtained by electrically processing the image.
However, the imagewise exposure with a digitally controlled light source from the interior of the electrophotographic member suffers from the following problem. Since the laser light or the LED light source produces monochromatic light, interference rings caused by reflecting light beams within the member are inevitably formed in the photoelectric photosensitive member or at the surface of the carrier adjacent the photosensitive layer, with the result that an irregular potential distribution is developed on the electrophotographic photosensitive member. This irregular potential distribution is critical particularly in image exposure, since image defects, known as interference fringes or rings, are produced by such an irregular potential distribution.
In order to prevent production of such interference fringes, U.S. Pat. No. 4,617,245 proposes to make coarse the surface of a carrier of a photosensitive member, which is made of a light-non-transmissive material, such as a metal. Meanwhile, U.S. Pat. No. 4,618,552 proposes to provide a light diffusion layer between the light-non-transmissive carrier and the photosensitive layer. These proposals, however, are applicable only to electrophotographic apparatus of the type in which image exposure is conducted by applying light from the exterior of the photosensitive member. If such proposals are applied to the apparatus of the type which employs image exposure from the interior of the photosensitive member, the copied image is undesirably disturbed because the light is scattered or diffused through the coarse surface of the carrier or the diffusion layer, with the result that the image quality is seriously degraded.